This invention relates to a circuit for synchronizing the horizontal frequency synchronizing pulse generator in a television camera system to a reference signal.
In television camera systems, it is necessary that the output of video signal be synchronized with an external reference so that the signals from various sources can be combined. A television camera typically consists of a camera head and a camera control unit. The camera head includes the precision optical mounts and mechanical assemblies, the image pickup tubes, and appropriate electronics to scan the image and produce video signals representative of the image. The control unit includes the electronics to process and encode the video signals produced by the camera head. The camera head and camera control unit are often interconnected by a cable of unknown length. This length may vary, for example, from 100 feet to as much as 2,000 feet. The phase delays introduced by this cable must be compensated for in order to achieve the desired horizontal synchronization.
In the past, the phase delay introduced by the cable has been compensated for by comparing the horizontal reference signal with the horizontal synchronization pulses received at the control unit from the camera head. The output of this comparator is a signal which is indicative of the phase error between the two horizontal synchronizing pulses. This signal is then utilized to control an oscillator or to control a controllable variable delay device situated in the control unit. This control loop will then automatically compensate for the phase delay of the cable.
It is also known to compare the vertical synchronizing pulse from the camera head with the vertical synchronizing signal to correct for errors in vertical synchronization, in a manner similar to that described above. Another previous technique employs an oscillator operating at 31.5 kilohertz, or twice the horizontal frequency. The output of this oscillator is divided by two to provide a 15.75 kilohertz horizontal synchronizing signal. The output of the oscillator is also divided by 525 to provide a 60 hertz vertical rate signal. This vertical rate signal is then compared with the 60 hertz power line frequency to develop a phase error signal. This phase error signal is utilized to control the 31.5 kilohertz oscillator to synchronize the system.
The cables utilized to interconnect the camera head and the control unit also introduces undesirable attenuation of the higher frequency signals transmitted along the cable. In order to assure a useful signal at the control unit end of the cable, the video signal transmitted from the camera head may utilize the maximum peak voltage of each polarity to represent black and white, respectively. A signal which alternates between the black and the white voltage levels may be inserted into the horizontal blanking interval for use by an equalizer circuit in order to compensate for this high frequency roll-off. It is desirable that the horizontal synchronizing pulse generator be locked in phase with the reference signal prior to the operation of the equalizer circuit so that the horizontal synchronizing pulse can be utilized to sample the inserted signal. It has been determined that it is also desirable that the horizontal oscillator phase control circuit utilize the same inserted signal as the equalizer circuit in order to avoid the necessity of inserting two signals into each horizontal blanking interval and the subsequent decoding problems. As a result of the fact that this inserted signal is of the same amplitude of the video signal and within the same frequency spectrum, the signal must be sampled during a period in which no video signal is present. It is desirable that no video signal be present before and after the inserted signal, in order to avoid an erroneous control signal. This criterion cannot be met by a system which samples during the horizontal blanking intervals.